To expand on Kev's note

Caused by misalignment of the scope and barrel. It's caused by mounts not being aligned or barrel being knocked out of true.

Simple check is to shoot a group at 20, 40, 50 yards and check. problem of course is wind - are you getting x over or wind?

If you do it indoors - I have access to 30 yards - the danger is that you get parallax error and so doing 15 and 30 yards needs to be carried out with a somewhat carefull eye.